1. Field of the Invention
This invention concerns a transparent coating for windows or other transparent substrates made of glass or synthetic resin, consisting of flexible elastic polyurethane that is prepared from a reaction mixture whose isocyanate component consists of essentially trifunctional aliphatic polyisocyanates with a biuret or isocyanurate structure, formed on the basis of 1,6-hexamethylenediisocyanate, with an NCO group content of 15 to 25% by weight and a molecular weight of approximately 450 to 1000 g per mole, whose polyol component contains at least one polyfunctional polyol, and that may contain a reaction catalyst and other auxiliary substances.
2. Description of the Prior Art
When coatings of the above type are applied to silicate glass windows, they serve as shatterproofing coatings that prevent contact with the sharp edges of glass fragments when the window is broken. The glass windows with such coatings are particularly suitable as safety glass for automobiles, and are well known, for example, from the published German patent application AS No. 2,228,299 and German Patent 2,452,928.
Coatings of the type indicated are also known as coatings for substrates made of transparent plastic materials (published German patent application OS Nos. 2,058,504, 2,634,816, and 3,201,849). In this case, these polyurethane coatings serve to improve the scratch resistance of the substrate. The flexible elastic polyurethane coatings actually have the property that under stresses of common types that lead to surface deformations and scratches with the harder synthetic resins, they experience only an elastic deformation that disappears after a short time. This property is also called the "self-healing property".
These known transparent polyurethane coatings can be applied to the substrate by various procedures. Thus, it is known how to first produce sheets of this material and to laminate these sheets to the substrate with an adhesive interlayer (published German patent application AS No. 2,629,779). It is also known how to pour or spray the reaction mixture reacting to form the polyurethane onto the substrate, in which case layers of an adhesion promoter may be applied to the substrate in advance.
The known coatings of the type indicated have remarkable properties with regard to their resistance to abrasion or scratch resistance, transparency, and other properties in use. However, they have the property that by condensation of atmospheric moisture on their surface, a deposit of very fine droplets of water is formed that can be detrimental to vision through the window or the sheet.